In the Trac Plugin documentation the configuration is described as

In the Source Code Management section, choose "Trac" for the "Repository browser" setting.

This settings "Trac" is not available for when I use git as a SCM.

Can this selection be added?

The Trac-plugin code supports "TracGit" for the "Repository browser" setting. The given Trac URL at the top of the job settings will be used to generate links for git changesets, files and diffs to your Trac "Browse Source" area.